Why Orange Walk? Agree Cayo is a better idea.
If Caracol is too much of a drive, don't forget Cahal Pech - easy to see, very interesting and with a super museum.
With the ages of your sons, maybe a couple of days at Ian Anderson's Caves Branch Adventure Camp -- what a fabulous place - varying levels of cost in accommodations, wonderful adventures and incredible food. (Yes I like it a lot).